Saw blades are subjected to different types of stress. A bandsaw blade is typically bent over the wheels at least twice per working cycle. The saw blade is tensioned prior to being put to use in order to compensate for the increased temperature during sawing which causes some expansion of the material. Each cut puts strain on each saw tooth. The resulting stress on the material is pulsating and the net average stress is tensile. This type of stress will often cause fatigue cracks in a material according to whether the stress cycle is contained within the Goodman diagram of the material in question or not. It is well known that by inducing residual stress in the material by deforming it, in other words by putting a strain exceeding the yield value on a small portion of the material, the resistance of the material to fatigue stress may be increased. Such application of residual stress is described in standard engineering texts and methods for deforming stressed parts of special products are described in e.g. U.S. Pat. No. 3,803,898 (1974) by E. R. Speakman, and SE 222317 (1959) by T. G. Angel. Both these methods are concerned with increasing the resistance to fatigue stress of the material around holes in sheet metal. Generally the successful application of such deformation is dependent on placing and grading the applied deforming force correctly, as otherwise reduced resistance to fatigue stress may result, if for example cracks are created already at this stage instead of being prevented from occurring later.
Saw blade gullet cracks are more frequent when the blades are not or are only seldom resharpened and when the notch is not filed when the tooth is resharpened. Obviously, small cracks in the gullets may be removed by filing and thereby the process leading to fatigue failure, which most often starts at the notch bottom, is stopped and must start afresh. Such saws which are not refiled at the notch are therefore well suited for the application of a deformation of the above described type before being put to use. The principal features of such deformation of saw blade gullets are outlined in the German published but not examined patent application DE 31 27 178A1. It is the object of this invention to provide a machine for carrying out such deformation at the exact place and with the exact amount of force needed in a simple and quick manner at a low cost per blade.
This object and other advantages and particulars of the invention will be apparent from the following description of the preferred embodiments and from the following patent claims.